Computer systems provide techniques for users to communicate with each other. One example system may make use of an Internet Protocol (“IP”) Multimedia Subsystem (“IMS”) core network (e.g., as standardized by the Third Generation Partnership Project (“3GPP”)), which may facilitate the use of IP protocols for communications, such as telephony, e-mail, voice over IP (“VoIP”), instant messaging, video messaging, and/or other services.
A service provider, such as a cellular telecommunications service provider, may provide an IMS core network, which may provide services to user devices that operate on a cellular network provided by the service provider. Other service providers may not provide an IMS core network, and user devices associated with these other service providers may lack IMS authentication functionality. Thus, when these user devices attach to networks that have IMS functionality (e.g., when roaming), these user devices may not be able to utilize IMS features.